In another page I showed how to use the SWR analyzer to calculate velocity factor for coaxial cable. As I mentioned you’d want to do this if you are installing some type of phasing harness or perhaps you have a radio but no tuner so you need to make sure you get a proper length feed line. What if things are acting up and you suspect that you have a problem (as in a short-circuit) in your cable? If it’s a long cable you might want to see if you can salvage some or all of it. To do that you would need to know where the short is so that you can excise the demon! How can you do that? Well, if you have a time domain reflectometer (TDR) it’s fairly easy to do. If not, you can use your SWR analyzer and some math. (I’ve provided a calculator to help with the math below)
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Using an SWR Analyzer to Find Coax Cable Short
I’ll outline the procedure, give you an example and the calculator.
Here’s the procedure:
- First disconnect the cable at both ends.
- Verify with an ohm meter that you truly have a short (already done, right?!)
- Connect one end of the cable to the your analyzer, leave the far end open
- Turn on your SWR analyzer at its LOWest frequency. As you sweep the frequency up, watch the resistance value, looking for 0Ω. Note that frequency.
- Continue raising the frequency, looking for a second low point of 0Ω. If you’ve done everything properly, the frequency of the dip should be twice of your first one. If it’s not, recheck looking for a dip at 1/2 of your second frequency and also looking for 2x the first. It may not be exact, but should be pretty close.
- Plug the frequency into the calculator below with your best guess at vf and you should get a reasonable number.
I mark this length with a piece of tape. I actually put a mark on the end I was working from (1 or 2, A or B) AND the same mark on the tape at the “location” of the short.
Being that I’m a “belt & suspenders” kind of guy or perhaps a member of the “measure twice cut once” carpenter’s club, I check from the other end of the cable. Doing exactly the same thing all over again, figure out the location of the short. If you’ve measured carefully and have the vf exactly right, the two pieces of tape will be on top of each other. If not, here are some possibilities to consider.
What do you mean the tapes didn’t line up?
- If the tapes crossed each other, meaning piece 1 is closer to end two AND piece 2 is closer to end one, your estimate of VF is too high.
- If the tapes did not cross each other, meaning piece 1 is closer to end one AND piece 2 is closer to end two, your estimate of VF is too low.
So what to do?
You could change your number for vf and recalculate the distances again and again until you get the tapes to line up on top of each other. I’m not sure how fast that will help you find coax cable short. But I have a simpler thing to do. You can scale your numbers by the amount you missed by. What do I mean? Let’s say you have a 100 ft piece of coax and your frequencies are 13.77 MHz and 4.59 MHz for the location of the short. You estimate the vf to be 0.6 so the distances to the short are: 21.4 feet and 64.3 feet. The two pieces of tape did NOT cross and they are still 14.3 feet apart. If you calculate a simple ratio, you can get a better guess as to where the short is actually located. The short will always be between the two pieces of tape, even if they crossed. It will also be closer to the shorter distance. The ratio to calculate is the shortest distance divided by the sum of the two distances. Multiply that be the difference in distance between the two pieces of tape, in this case 14.3 ft. In this case the equation is
- 21.4 / ( 21.4 + 64.3) => 0.25;
- so the delta is 0.25 * 14.3 ft = 3.5 ft change towards the other mark, in this case continuing in the same direction;
- Lastly: original length + delta = 21.4ft + 3.5ft = 24.9 ft
Let’s say you did the exact same thing only this time used a vf of 0.9: the distances become 32.1 ft and 96.4 ft and the two pieces of tape did pass each other (28.5 ft)
- 32.1 / ( 32.1 + 96.4 ) => 0.25;
- and 0.25 * 28.5 = 7.1 ft change towards the other mark, in this case doubling back towards the start;
- Lastly: original length – delta = 32.1 – 7.1 ft = 25.0 ft
For this example, the actual vf was 0.7 and the short was at 25 ft from the closer end.
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One caution to remember when using an SWR analyzer to find coax cable short-circuit is that we are assuming a single, distinct short. In reality, you could have a cable with water in it over a several foot distance or other non-distinct failure or perhaps even multiple failures. In some cases, inspecting the exterior of the cable may give you some clues as to the exact location of the short.
If you have to the math manually because your browser doesn’t support iframes, or you just want to understand, the formula for the distance to find coax cable short-circuit location is:
distance (feet) = 492 * vf / f (MHz of the low impedance)
The formula should look very familiar as in the half-wave dipole formula since that’s all we’re doing is looking for the length to achieve the zero.
Please let me know what you think by commenting below.